1. Field of the Invention
This invention relates to a photoelectric device having a light transmitting section and a light receiving section disposed remote from each other, and more particularly to a photoelectric detector of this type which detects cut-off or attenuation of transmitted light beams due to intervention of smoke etc. in space between the light transmitting and receiving sections, by synchronizing the operation of the light receiving section with the operation of the light transmitting section.
2. Description of the Prior Art
In general, to measure a concentration of smoke or transmission of light in space, light from a light transmitting section, having a light source, is detected by a light receiving section to obtain a transmission or extinction of the light between the light transmitting and light receiving sections. However, the light receiving section is subject to various disturbing foreign light, such as modulated light, (as from a fluorescent lamp nearby), incident thereto as well as the light from the light source of the light transmitting section and, as is widely known, an accurate light transmission or extinction cannot be measured unless such foreign light is eliminated.
Heretofore, there have been proposed, to solve this problem, (1) a method wherein light radiated from the light transmitting section is modulated by a mechanical chopper and the modulated light is demodulated by the light receiving section, (2) a method wherein the light source of the light transmitting section is driven, for example, at 1 KHz and the received light signal is treated through a filter of 1 KHz, and (3) a method wherein the light source of the light transmitting section is intermittently driven at a modulated frequency and a signal, synchronized with the intermittent drive of the light source, is supplied to the light receiving section so as to allow the light receiving section to treat only received light signals synchronized with the intermittent drive of the light source.
However, the conventional method (1) has such as a disadvantage that an apparatus for the method is generally expensive and not durable enough for long use because of the mechanical element employed in it, and the conventional method (2) also has as a disadvantage that the signal-to-noise ratio (SN ratio) of the received light signal to the foreign light is not satisfactory because high frequency components of the foreign light are possibly contained within a passband of the filter. In contrast, the conventional method (3) has a relatively excellent SN ratio of the received light signal to the foreign light since the modulated light is transmitted intermittently and the light receiving section receives the light in synchronism with the modulated light. Thus, this method has been widely used. However, where this method is applied to a fire sensor of a fire alarm system, the light transmitting and light receiving sections are supplied with a power source from a receiver of the fire alarm system and a special signal line or lines are additionally required to transmit and receive synchronizing pulses. Thus, the method (3) has a problem in installation of the lines. Since a separate type fire sensor is generally installed in a warehouse etc., and the light transmitting section is separated from the light receiving section by anywhere from several meters to several hundreds meters, such increases in the number of the lines provide a serious problem.